I think the danger with underpitching is that the longer lag time 'till the yeast gets going can mean more risk of an infection, and the yeast could also produce off flavours. Do you normally make a starter with your liquid yeast? I would think making a bigger starter and starting it a day earlier to give the yeast more time to reproduce would give you enough for a 10g batch. I've also seen threads on here about doing a couple of steps to work up to a starter that's big enough for 10g.
I guess it partially depends if you have the activator or propagator size of yeast. The activator claims to be enough to pitch directly into 5g (though some would debate this), so I'm sure you could pitch it right into a 2+ liter starter. With the smaller propagator, I might do a day in a 1 liter starter, then move it up to a larger one for another day before pitching into your beer.